1. Field of the Invention
The present invention is directed to ink-jet compositions particularly solid ink-jet compositions for use in thermal ink-jet printers. Specifically, the present invention is directed to solid ink-jet compositions for use in thermal ink-jet printing using thermally induced vapor bubbles which compositions are formulated so as to provide improved printing characteristics including greater control of the solid ink dot height on the absorbing surface, i.e. the substrate.
2. State of the Art
In thermal ink-jet printing, solid ink-jet compositions are typically employed. These compositions typically employ an ink which is normally a solid at room temperature and which is capable of a phase change at elevated temperatures. When the ink is heated, the ink melts to form a fluid which can be ejected from the printer as droplets onto a substrate for marking whereupon it resolidifies by freezing on the marked substrate. Because of their nature of operation, such ink compositions are termed "hot melt inks" which term is art recognized to define an ink which is in a solid phase at room temperature and in a fluid phase at the operating temperature, i.e., a temperature above the melting temperature of the waxy component of the ink. Most solid ink-jet compositions reported so far have involved one phase change, that is, from solid at room temperature to liquid at operating temperatures.
Solid ink-jet compositions can be used in thermal ink-jet printing utilizing thermally induced vapor bubbles as a driving force to eject ink drops out of the nozzle of the jet printer and onto the substrate. When so used, the composition is sometimes referred to herein as a thermal ink-jet ink composition. In this case, a portion of the solid ink in the printing head is first liquified and then a portion of the liquified ink is vaporized so as to generate a bubble which is used to force the ink from the printer head and onto the substrate.
Thermal ink-jet ink compositions generally contain a carrier, a driver and a colorant. The carrier is an organic material which carries the colorant(s) and which is a solid at 25.degree. C. and a liquid at the operating temperature of the printer head. The driver is a bubble-forming substance which can provide enough driving force to eject ink drops from the print head or nozzle. The colorant can be a dye or pigment, which is soluble in the carrier and which produces the visible printed images on the substrate. Upon contact with the surface of the substrate, the ink composition rapidly freezes to form an ink dot which imparts a mark onto the substrate.
However, with regard to such solid ink compositions, a problem exists with regard to the formation of raised dots of ink droplet forming on the substrate surfaces. These raised dots arise from the instant freezing of the ink droplet on the substrate surface and the resulting lack of spreading of the droplet on the surface. The raised dots are aesthetically unacceptable since they can result in images with an embossed characteristic. Additionally, when the raised ink dots becomes too high, the raised dot may have poor adhesion to the substrate or may easily be scraped off or flake off from the substrate by action of folding or creasing of the substrate or may be subject to smearing or offsetting to other sheets.
The reduction of the height of such raised dots can be achieved by incorporating a sufficient amount of a penetrate into the ink composition. However, the use of such penetrates can result in the dye in the ink being carried unevenly into the pores of certain substrates, e.g., paper, by capillary action which in turn leads to feathering, i.e., the undesired diffusion of the dye which results in the dot becoming fuzzy. This, in turn, results in poor print quality.
Other heretofore known means for reducing dot height include the procedures disclosed in U.S. Pat. Nos. 4,745,420: and 4,801,473: and European Patent Application Pub. No. 308 117A. In particular, U.S. Pat. No. 4,745,420 discloses passing the marked substrate through rollers in order to minimize the height of such raised dots. Likewise, U.S. Pat. No. 4,801,473 discloses the use of a liquid coating applied to the surface of the marked substrate which wets the surface of the substrate as well as the ink dots. The coating apparently reduces dot height by increasing the surface area of the dot. Also, European Patent Application Pub. No. 308 117A discloses that the initially formed dots in a solid state can be flattened by heating the substrate and then subsequently cooling it. However, as can be seen, each of these other procedures requires a separate post-treatment step to reduce dot height which imparts an additional level of complexity to the printing process.
In View of the above, solid ink-jet compositions formulated to reduce dot height without resulting in feathering would provide an important advance in the art of thermal ink-jet printing.
In this regard, the present invention is directed to the discovery that solid ink-jet compositions meeting defined criteria including a specified surface tension range and a defined viscosity range permit the application of printed dots of reduced dot height without causing feathering provided that the printer head is operated at defined temperature differentials from the melting point of the ink and from the temperature of the substrate.
Specifically, the present invention is directed to the discovery that in thermal ink-jet printing utilizing thermally induced vapor bubbles, solid ink-jet compositions which are formulated to have less than a specified maximal surface tension will result in reduced dot height as compared to the dot height resulting from the use of ink compositions having a surface tension greater than that described herein. Likewise, when the herein described compositions are also formulated to have greater than a specified minimal surface tension, then the resulting ink composition achieves such reduced dot height without causing feathering of the dot into the substrate.
The above discovery is particularly surprising in view of the fact that, in oil based inks, ink compositions with too low of a surface tension result in poor print quality. See, for instance, Lin et al., U.S. Pat. No. 4,758,276. Further in this regard, Cooke et al., U.S. Pat. No. 4,361,843, discloses that acceptable print quality is achieved by oil based inks employing oleic acid when the surface tension of the ink is greater than 35 dynes per centimeter.
With regard to the defined criteria concerning viscosity and the defined temperature differentials, the present invention is directed to the discovery that these criteria are also essential to achieving reduced dot height.